Haemophilus ducreyi, the etiologic agent of the genital ulcer disease chancroid is a significant public health problem in several regions worldwide. In developing countries of Africa, Asia and other continents, it is an important co-factor for the heterosexual transmission of HIV. Control of chancroid, using an effective vaccine that is properly administered to the core group of commercial sex workers that serve as a reservoir, would likely reduce HIV transmission in the general population. It is the goal of my laboratory to develop such a vaccine. We propose to continue our vaccine studies on the hemoglobin receptor (HgbA) of H. ducreyi. Several attributes of HgbA make it an attractive vaccine candidate. HgbA is required to establish human experimental infection. HgbA is conserved immunologically and functionally and all virulent H. ducreyi strains express it. Unlike hemoglobin receptors from other bacteria, HgbA does not undergo phase or antigenic variation. Our recent studies in the swine model of chancroid infection, that closely resembles natural human infection, demonstrate that purified native HgbA is a highly effective vaccine. Additional vaccine studies proposed here will be undertaken using recombinant (rHgbA). We will test the ability of HgbA vaccines to protect against homologous and heterologous challenge infections using various non-toxic adjuvants. We will begin to address the mechanism of protection by the HgbA vaccine. We will perform classical passive transfer of IgG to formally prove that the protection observed is antibody-mediated. We will determine role of neutrophils in the killing of H. ducreyi opsonized with HgbA antibodies, identify the domain(s) of HgbA that are necessary for binding its ligand human hemoglobin, and identify regions of HgbA that elicit bactericidal antibodies. These studies are critical to rational vaccine development for chancroid.